I assume the person you used isn't in a competent person scheme like NICEIC or NAPIT? By rights they should be if they are changing domestic consumer units as the work should be notified to building control. You should also have a certificate proving they tested it all and it's safe.
The original 18th edition (from Jan 2019) required domestic circuits supplying luminaires to have RCD protection.
It's reg 411.3.4 and it's in the blue book. So this argument doesn't hold water at all.
Here's a web page that was released when the 18th edition first came out that mentions the change.
EXCERPT OO [...]Read More...
www.tradeskills4u.co.uk
The problem is that I'm not sure I'd want them to fix the issue anyway from what you've said.
It might be that RCBO's can be fitted for the lighting to avoid changing the whole board. But sometimes this isn't possible or cost effective.
I think you need to get a decent sparks around to have a look at it and give you some advice.